The identification of the cell junction-forming proteins connexin-43, a gap junction protein and E-cadherin, which is a component of adherent junctions, in the corpus luteum of both humans and baboons suggests that cell-cell interactions and metabolic cooperation must occur in this tissue. Occluding junctions are a third type of junction which form a physical barrier between cells. Thus, our aims in this study were firstly to examine the presence of the tight junction-associated protein zonula occludens-1 (ZO-1) by immunohistochemistry, and secondly to determine the concentrations of this protein in the early-, mid- and late luteal phase baboon corpora lutea of the menstrual cycle by a Western analysis. ZO-1 was localized mainly at the periphery of the luteal cells, and the intensity of immunoreactivity varied through the luteal phase, with comparatively stronger immunoreactivity in the mid-luteal phase than the early and late luteal phases. Atretic corpora lutea were devoid of activity. By Western analysis, bands of immunoreactivity were observed at 225 kDa, further confirming the presence of the protein. Maximum activity, as determined by densitometry, was observed in the mid-luteal phase. These data infer the presence of tight junctions in the corpus luteum and suggest that expression of the ZO-1 protein forming these junctions may be hormonally regulated within this tissue.